Efforts are being made to save fossil fuels or to apply renewable energy to more fields by improving the use efficiency to solve the problem of depletion of fossil fuels and environmental pollution.
Renewable energy sources such as solar heat and wind power have been used more efficiently than before, but these energy sources are intermittent and unpredictable. Due to these characteristics, their dependence on these energy sources is limited, and a ratio of renewable energy sources to the current primary power sources is very low.
Since a rechargeable battery provides a simple and efficient method for storing electric power, the rechargeable battery have been miniaturized to increase its mobility, and efforts to utilize the rechargeable battery as power sources for small home appliances such as an intermittent auxiliary power source, a laptop, a tablet PC, and a mobile phone have continued.
Among them, a redox flow battery (RFB) is a secondary battery capable of storing energy for a long time by repeating charging and discharging by an electrochemical reversible reaction of an electrolyte. A stack and an electrolyte tank, which depend on the capacity and output characteristics of the battery, are independent of each other, so that a battery design is free and limitation of an installation space is small.
In addition, the redox flow battery has a load leveling function installed in a power plant, a power system, and a building to cope with an abrupt increase in power demand, a function of compensating or suppressing a power failure or an instantaneous undervoltage, and the like. The redox flow battery is a very powerful storage technology capable of being freely combined if necessary, and a system suitable for large-scale energy storage.
The redox flow battery generally consists of two separated electrolytes. One electrolyte stores an electric active material in an anode reaction and the other electrolyte is used for a cathode reaction. In an actual redox flow battery, the electrolyte reaction is different between the cathode and the anode and there is a flow phenomenon of the electrolyte solution, so that a pressure difference occurs between the cathode side and the anode side. In an all vanadium-based redox flow battery as a representative redox flow battery, reactions of the cathode and anode electrolytes are shown in the following Reaction Formulas 1 and 2, respectively.
                              VO                      2            +                          ⁢                  ⇄          discharge          charge                ⁢                  VO                      2            +                                              [                  Reaction          ⁢                                          ⁢          Formula          ⁢                                          ⁢          1                ]                                          V                      2            +                          ⁢                  ⇄          charge          discharge                ⁢                  V                      3            +                                              [                  Reaction          ⁢                                          ⁢          Formula          ⁢                                          ⁢          2                ]            
Therefore, in order to overcome the pressure difference between the both electrodes and to exhibit excellent cell performance even if charging and discharging are repeated, an ion exchange membrane having improved physical and chemical durability is required. In the redox flow battery, the ion exchange membrane is a core material accounting for about 10% of the system.
As such, in the redox flow battery, the ion exchange membrane is a main component for determining the lifespan and price of the battery. In order to commercialize the redox flow battery, a low crossover of vanadium ions is required due to high ion selective permeability of the ion exchange membrane, high ion-conductivity is required due to low electrical resistance, and a low price is required in addition to mechanical and chemical stability and high durability.
Meanwhile, currently, polymer electrolyte membranes commercialized as ion exchange membranes have been used for tens of years and have been continuously studied. Recently, as a mediator that transfers ions used in a direct methanol fuel cell (DMFC), a polymer electrolyte membrane fuel cell (proton exchange membrane fuel cell, PEMFC), a redox flow battery, water purification, and the like, many studies on the ion exchange membrane has been actively conducted.
Currently, a widely used material for the ion exchange membrane is a Nafion™-based membrane, which is a perfluorinated sulfonic acid group-containing polymer manufactured by DuPont in USA. At a saturated moisture content, the membrane has ion-conductivity of 0.08 S/cm at room temperature and excellent mechanical strength and chemical resistance and has stable performance as an electrolyte membrane for use in automotive fuel cells. Further, as similar types of commercial membranes, there are an Aciplex-S membrane from Asahi Chemicals, a Dow membrane from Dow Chemicals, a Flemion membrane from Asahi Glass, a GoreSelcet membrane from Gore & Associate, and the like. In the Ballard Power System, Canada, alpha or beta types of perfluorinated polymers have been developed and studied.
However, the membranes have disadvantages of not only having a difficulty in mass production due to a high price and a complicated synthesis method but also greatly lowering efficiency as the ion exchange membrane such as a crossover phenomenon and low ion-conductivity at a high temperature or a low temperature in an electric energy system such as a redox flow battery.